Using only a standard NE555 timer and a few other parts, this circuit should be up an running in about an hour. The 555 is configured as a minimum parts count astable (i.e. no stable states) multivibrator, and runs at around 17kHz with the values shown. The zener diode (D3) should be a 16V/ 1W type. Resistors are 1/2W carbon film, and small caps may be polyester or mylar (63V types are quite Ok in this circuit).
Use a standard (bipolar) NE555 timer - not a CMOS type. CMOS timers do not have the switching abilities of the bipolar types, and output voltage will be lower.
The circuit itself is a simple voltage doubler. You may well ask why the output is not -24V if the circuit is a doubler. Look at the circuit, and you will see that the output of the timer is AC coupled with C4, so it is actually only 6V RMS with a 12V supply. For an input of 13.8V (standard car voltage), the doubler action gives an output of -12.5V with no load. There is a small loss due to the diode losses, and additional losses in the 555´s transistor switched output.
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